A record of estuarine water contamination from the Cd content of foraminiferal tests in San Francisco Bay, California

A five-year dissolved Cd time series from San Francisco Bay and adjacent coastal water shows that the composition of surface water towards the mouth of the estuary is determined largely by the effect of coastal upwelling. Cd concentrations Ž . inside and outside the estuary 0.2–1.0 nmolrkg increase as Cd-rich deep water is advected to the surface near the coast Ž . during spring and summer. On average, the mean Cd concentrations inside San Francisco Bay 0.54 nmolrkg during Ž . 1991–1995 was significantly higher than outside 0.35 nmolrkg , however. Surface samples collected throughout San Francisco Bay confirm an internal Cd source unrelated to river discharge. The Cd content of the test of a benthic foraminifer Ž . Elphidiella hannai in a dated sediment core from San Francisco Bay was measured to determine if the water column Cd enrichments in San Francisco Bay could be related to the rapid development of the watershed. The method is based on the observation that the CdrCa ratio of carefully cleaned tests of foraminifera is determined by the dissolved Cd content of overlying water at the time of test formation. Pre-industrial foraminiferal CdrCa ratios in the sediment core average Ž . Ž . 274"15 nmolrmol ns19 nmolrmol. Foraminiferal CdrCa ratios increased to 386"33 nmolrmol ns19 over the past several decades indicating a 40% increase in the mean Cd content of surface water in Central San Francisco Bay. We suggest that, in addition to Cd discharges into the estuary, indirect consequences of agricultural development in the Central Valley of California could have contributed to this increase. This new method to reconstruct estuarine contamination is not affected by some of the processes that complicate the interpretation of changes in bulk sediment metal concentrations. q 1999 Elsevier Science B.V. All rights reserved.